This grant proposes the development of a high yield neonatal serotonin screening test to detect a group of diseases causing mental retardation by a single chemical determination. This new test is based on a different scientific approach than present neonatal testing of multiple disease entities. One sample of blood for the endogenous level of serotonin taken by heel stick may detect the following diseases: Low levels: PKU, histidinemia, Down's syndrome, Cornelia deLange syndrome; High levels: Infant hypothyroidism, infantile spasm syndrome with retardation, high serotonin syndrome with retardation, infantile autism, "cerebral palsy" when retardation is present, maternal rubella, kernicterus. Treatments designed to ameliorate or correct mental retardation are available in some of these disease entities. This is the only screening test routinely detecting cretins. The explanation of why the platelet serotonin level is useful in neurological diagnosis lies in the concept of a partial functional model system. The platelet appears to be a partial model for the serotonergic neuron. Both laboratory and clinical evidence have shown that similar factors effect active transport and intracellular binding of serotonin in the platelet and in the serotonergic neuron. The proposed serotonin screening test hopes to utilize this relationship for effective early screening of neonates. An incomplete pilot study shows detection of 1.6% from a normal nursery. In each patient one sample of blood taken by heel stick will be analyzed for total 5-hydroxyindoles by a modified fluorometric procedure. The patients will be infants admitted to the Columbia Hospital for Women in Washington, D. C.